The present invention relates to improved poly-p-phenylene-terephthalamide (hereinafter referred to as "PPTA") fibers and a process for their preparation. More particularly, the invention relates to high tenacity, PPTA fibers showing excellent fatigue resistance when used for reinforcing plastics and rubbers, and a process for their preparation.
PPTA is a polymer that has been known for many years and, from the rigid molecular structure of this polymer, it has been expected that its fibers would have excellent heat resistance and mechanical properties. However, PPTA is only slightly soluble or insoluble in organic solvents. Accordingly, Cypriani proposed a basic process for wet-spinning PPTA by using concentrated sulfuric acid as a solvent (U.S. Pat. No. 3,227,793), but Cypriani's process itself was not industrialized.
It has been known for many years that when a rigid polymer is dissolved in a solvent, a liquid crystal is formed at a degree of polymerization exceeding a certain level and a concentration exceeding a certain level under a certain temperature condition, and this phenomenon has been confirmed theoretically and experimentally (P. J. Flory; Proc. Roy. Soc., A234, 73 (1956)). It is easily predicted that if a polymer solution that is in the form of a liquid crystal and is optically anisotropic can be extruded from a nozzle and coagulated without disturbing orientation of the liquid crystal caused in the interior of the nozzle, fibers having high tenacity and high Young's modulus and comprising highly oriented molecular chains will be obtained. In fact, Kwoleck proposed a process for the wet spinning of a concentrated solution of an aromatic polyamide having a rigid and linear molecular structure that is in the form of a liquid crystal (U.S. Pat. No. 3,819,587), and this type of wet spinning process again attracted attention in the art.
Even if the process of Kwoleck is adopted, however, to obtain a valuable high strength and tenacity, the as-spun fibers should be subjected to heat treatment under tension. According to this heat treatment under tension, the tenacity is improved and the Young's modulus is increased, but the elongation is reduced. Accordingly, these fibers are not satisfactory in fatigue resistance when they are used for reinforcing rubber articles such as tires.
Blades proposed a process in which an optically anisotropic dope having an elevated concentration is extruded in the air and then wet-spun to form as-spun fibers having a high tenacity owing to a specific fine-structure in the as-spun state (U.S. Pat. Nos. 3,767,756 and 3,869,429), and Blades further taught that these fibers are suitable for reinforcing rubbers.
Some of the inventors of the present invention noted that, since Blades specified the fine-structure of the fiber only with respect to the crystalline region thereof, the desirable effect is not actually attained by the fine-structure specified by Blades and found that, even if the teachings of Blades are precisely traced, good reproduction in the relations between physical properties and the fine-structure specified by Blades cannot be obtained. Accordingly, some of the inventors of the present invention conducted research and found that it is not reasonable to note the crystalline region alone according to the position of Blades, and also, that the amorphous region should be taken into account in defining fibers having desirable properties. Based on this finding, research was further conducted on the relation of the fine-structure of not only the crystalline region, but also the amorphous region in PPTA fibers to the characteristics of the fibers. As a result, the concept of the present invention was formed.
The remainder of us conducted research on the relation between the fiber preparation process and the fiber characteristics, and have already proposed several processes for wet spinning of optically anisotropic dopes of rigid polymers having a linear structure, for example, a process in which an optically anisotropic dope of an aromatic polyamide is wet-spun, and water-washing finishing of the coagulated fibers is carried out in the absence of substantial tension. It was already disclosed that according to this process, reduction of elongation is prevented and fibers showing an excellent fatigue resistance when used for reinforcing rubbers can be provided (the non-heat-treatment process in U.S. Pat. No. 4,016,236).
PPTA, which is a typical instance of rigid and linear aromatic polyamides, is advantageous in that starting monomers having high quality are readily available and this polyamide can be used very easily on an industrial scale. Since PPTA is readily oriented and crystallized in preparing fibers from PPTA, fibers prepared according to the process disclosed in the non-heat-treatment process in U.S. Pat. No. 4,016,236 readily undergo changes of physical properties, such as reduction of elongation, when they are exposed to slight tension or heat at necessary steps for reinforcing rubbers, such as the adhesive treatment, rubber vulcanization and other post-treatment. It is presumed that the reason for this is probably that thermal setting of the fine-structure of fibers is insufficient in the above-mentioned process disclosed in the non-heat-treatment process in U.S. Pat. No. 4,016,236. In fact, when fibers prepared according to this process are used for reinforcing rubbers, no satisfactory results are obtained with respect to fatigue resistance.
Furthermore, a process for preparing thermally set fibers by conducting water washing, drying, and heat-treatment in the absence of tension was introduced in the heat-treatment process in U.S. Pat. No. 4,016,236. However, when this technique is applied to PPTA, it is very difficult to control the parameters of the fine-structure of fibers, such as the crystallite size, the degree of crystallinity, the degree of orientation of molecular chains in the crystalline region, and the degree of orientation of molecular chains in the amorphous region. Accordingly, although fibers prepared according to this process are substantially uniform in mechanical properties, such as elongation, tenacity, and Young's modulus, there is observed a considerable variation in the fatigue resistance when they are used for reinforcing rubbers.
Methods for preparing fibers excellent in fatigue resistance by improving PPTA fibers disclosed in U.S. Pat. No. 3,869,429, etc., while retaining excellent mechanical properties, such as high Young's modulus and high tenacity, are roughly divided into two types. More specifically, one type is based on the technique of changing the chemical structure and the other type is based on technique of changing the fine-structure of fibers without changing the chemical structure. As an instance of the former type, there was proposed a process in which improvements are attained by copolymerization (see, for example, Japanese Patent Application Laid-Open Specification 116322/74 and Yabuki et al, Sen-i Gakkai Shi, 34, T187 (1978)). This process, however, is fatally defective in that an increase of the manufacturing cost by the copolymerization step cannot be avoided when the process is worked on an industrial scale. The present invention belongs to the latter type and is based on the technique of changing the fine-structure of fibers. We succeeded in preparing PPTA fibers having a highly improved fatigue resistance by improving the preparation processes proposed in the past by us in U.S. Pat. No. 4,016,236.